Devices for protecting motor vehicles usually referred to as electronic anti-starting systems, comprise a central unit mounted on the vehicle comprising a transmitter/receiver circuit associated with antennas adapted to cover the entire volume of the cabin, and a tag carried by an authorized person, the said central unit and the electronic tag being programmed in such a way as to implement an authorization procedure according to which the central unit instructs the emission of an identification request signal on receipt of which the electronic tag is adapted to transmit in response an identification signal identifiable by the said central unit.
One of the advantages of such protection devices resides in the elimination of the key conventionally used to undertake the starting of the vehicle, which is replaced with an electronic tag simply carried by an authorized person.
However, against this advantage, these protection devices constitute a weak point in terms of security against the theft of the vehicles. Specifically, by virtue of their very design, they require that a middle-distance bi-directional dialogue be instigated between the vehicle and the electronic tag, instead of the short-distance dialogue instigated between a conventional key and an antenna disposed on an antitheft contactor. However, an increase in distance such as this is equivalent to offering the possibility of installing, between the vehicle and the electronic tag, a radio relay undetectable by the protection device, allowing recognition of the electronic tag at very long distance. Stated otherwise, on account of this increase in distance, it turns out to be possible for a malicious person to undertake the starting of the vehicle without being in possession of the electronic tag.
With a view to alleviating this drawback, a current solution consists in varying the field level and/or the orientation of the magnetic field emitted by the central unit of the vehicle destined for the electronic tag, and in programming this tag in such a way that it conditions the emission of an authorization to start on the prior detection of a variation of the magnetic field received.
However, once this principle of protection is known, it turns out to be easily reproducible; specifically, the amplitude of the programmed variation may not be measured absolutely, and it therefore suffices to design a radio relay able to reproduce an alteration in the magnetic field, amplitude or orientation, consisting of a simple image of the programmed variation of the magnetic field.
The present invention aims to alleviate this drawback and its main objective is to provide a method and a device for protection offering very elevated conditions of tamper-proofing in relation to a possible hijacking attempt.